This application is based upon and claims priority of Japanese Patent Application No. 2001-315681 filed in Oct. 12, 2001, the contents being incorporated herein by reference.
The present invention relates to a high temperature superconductor film and a method for forming the same, and a superconductor element using the high temperature superconductor film, more specifically to a Yxe2x80x94Baxe2x80x94Cuxe2x80x94O-based high temperature superconductor film and a method for forming the same, and a superconductor element using the high temperature superconductor film.
It is known that oxide high temperature superconductor has much lower surface resistances at high frequencies in comparison with electric conductors, such as copper, etc. For example, Yxe2x80x94Baxe2x80x94Cuxe2x80x94O-based high temperature superconductor film (hereinafter called also xe2x80x9cYBCO-based high temperature superconductor filmxe2x80x9d) can have surface resistances at, e.g., 2 GHz, which is lower by above two-digits than copper.
In view of this, it is expected to use the YBCO-based high temperature superconductor film as a circuit pattern to thereby provide a superconducting filter having a small insertion loss, a sharp frequency cut-off characteristics and a high unloaded Q. An unloaded Q value means an inverse number of a loss.
A superconducting filter having a small insertion loss, sharp frequency cut-off characteristics and a high unloaded Q is used in communication apparatuses and equipmenets of base stations of mobile communication, whereby the efficiency of using frequencies can be improved (Reference: Toshio NOJIMA and Kei SATOH, OYO BUTURI (Applied Physics), Vol.70, No.1, p.28-32 (2001)).
A method for forming the conventional YBCO-based high temperature superconductor film will be explained with reference to FIG. 6. FIG. 6A is a sectional view of the conventional YBCO-based high temperature superconductor film (Part 1). FIG. 6B is a sectional view of the conventional YBCO-based high temperature superconductor film (Part 2).
First, a dielectric substrate 110 of, e.g., MgO single crystal is placed in a film forming chamber of a pulsed laser deposition (PLD) system.
Then, a YBCO-based high temperature superconductor film 112 is grown on the entire surface of the dielectric substrate 110 by pulsed laser deposition. As a target is used a target of a composition which is as approximate to a stoichiometric composition (Y:Ba:Cu=1:2:3) of the YBCO-based high temperature superconductor film as possible, specifically to a sintered body of, e.g., Y1Ba2Cu3Ox.
The thus-grown YBCO-based high temperature superconductor film 112 is oriented in the direction of c axis with respect to the plane of the dielectric substrate 110, i.e., the YBCO-based high temperature superconductor film 112 is oriented in the longitudinal axis of the lattice.
However, the conventional YBCO-based high temperature superconductor film 112 does not have good quality. That is, the conventional YBOC-based high temperature superconductor film 112 shown in FIG. 6A has crystal defects (crystal dislocations) 113, crystal strains (not shown), etc. caused in the film. The conventional YBCO-based high temperature superconductor film 112 shown in FIG. 6B has yttrium oxides 115, such as Y2O3, Y2Ba1Cu1O5, etc. produced in the film. Pores 117 are generated near the yttrium oxides 115.
A circuit pattern of a superconducting filter is formed by patterning a high temperature superconductor film in a required configuration. Accordingly, when the high temperature superconductor film 112 as shown in FIG. 6 is patterned to form a circuit pattern, the crystal defects 113, and concavities and convexities are exposed on the side surfaces of the circuit pattern, or non-superconducting phases, as of yttrium oxides 112, etc., are exposed on the side surfaces of the circuit pattern. In the circuit patterns of filters formed of high temperature superconductor films, high-frequency signals tend to propagate along the side surfaces of the circuit patterns. Accordingly, when the circuit pattern has the crystal defects 113, and concavities and convexities exposed on the side surfaces, or non-super conducting phases, as of yttrium oxides 112, etc., exposed on the side surfaces, surface resistances at high frequencies are high.
Thus, when a circuit pattern is formed of the conventional YBCO-based high temperature superconductor film 112 as shown in FIGS. 6A and 6B, it is difficult to make a superconducting filter having a small insertion loss, sharp frequency cut-off characteristics and high unloaded Q.
An object of the present invention is to provide a YBCO-based high temperature superconductor film of good film quality and a method for forming the same, and a superconductor element using the YBCO-based high temperature superconductor film and having good electric characteristics.
According to one aspect of the present invention, there is provided a high temperature superconductor film which is Yxe2x80x94Baxe2x80x94Cuxe2x80x94O-based and formed on a dielectric substrate, a Cu composition ratio to the Ba near the upper surface of the film being higher than a composition Cu ratio to the Ba inside the film.
According to another aspect of the present invention, there is provided a method for forming a high temperature superconductor film comprising the step of growing a Yxe2x80x94Baxe2x80x94Cuxe2x80x94O-based high temperature superconductor film on a dielectric substrate, in the step of growing the high temperature superconducting film, the high temperature superconductor film is grown with a Cu composition ratio to the Ba near the upper surface of the film kept higher with respect to a Cu composition ratio to the Ba inside the film.
According to farther another aspect of the present invention, there is provided a superconductor element comprising a Yxe2x80x94Baxe2x80x94Cuxe2x80x94O-based high temperature superconductor film, a Cu composition ratio to the Ba near the upper surface of the film is higher than a Cu composition ratio with respect to the Ba inside the film.
As described above, according to the present invention, the YBCO-based high temperature superconductor film is formed with a Cu composition on the film surface maintained higher with respect to the stoichiometric composition, whereby the Cu oxide can be easily produced while the production of the yttrium oxides can be depressed. According to the present invention, the yttrium oxides cannot be easily produced, which makes it difficult for pores and crystal strains to be generated while the Cu oxide functions as a flux for advancing the crystal growth, whereby the YBCO-based high temperature superconductor film can have good film quality and single crystal. According to the present invention, the superconducting elements can be formed of the YBCO-based high temperature superconducting film of such good film quality.